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    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script type="text/javascript">
        var x=10;
        function fn()
        {
            console.log(x);
        }
        function show(f)
        {
            var x=20;
            f();
        }
        show(fn);//局部作用域没有定义x，所以从全局作用域寻找x，最终输出x=10
    </script>
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